Quoted:
It is because of all the mail/freight lost on 911.
Ups usps fedex etc,do not use commerical airliners anymore .
Instead,they have shared usage of their own planes.
No.
FedEx and UPS have their own separate fleets. USPS has always contracted with other carriers for it's air network, it has never owned it's own fleet. Since August of 2001, FedEx has had the contract to fly the majority of USPS priority mail. UPS has a small portion. That's why you see FedEx boxes outside of your post office, part of the partnership.
Post-9/11, priority mail does not fly on commercial (passenger) aircraft, due to concerns about terrorist activity, namely explosives. First class mail is still flown on commercial planes.
USPS is required by law to deliver to any address, whereas UPS and FedEx are not. Many times it is cheaper for FedEx and UPS to contract delivery to USPS, especially in rural areas. And yes, it usually takes much longer because all the companies involved tend to treat such shipments as low-priority stuff.